1. Technical Field
The present disclosure relates to water splitting systems. In particular, the present disclosure relates to hydrogen generation systems in which a light source is used for the photocatalytic decomposition of water.
2. Background Information
Because a chemical energy carrier offers the only practical means for storing large amounts of energy, hydrogen is a primary candidate for future energy storage. Although many methods exist for the production of hydrogen, most of those methods have problems regarding production efficiency and costs.
Photoelectric materials are candidates for an efficient method for producing hydrogen. These methods generally require strong UV/visible light absorption; high chemical stability in the dark and under illumination; suitable band edge alignment to enable reduction/oxidation of water; efficient charge transport in the semiconductor; and low over potentials for the reduction/oxidation reactions.
One attractive technology for producing hydrogen employs photo-electrochemical devices (PEC cells) for water splitting—cleaving water molecules into their components, hydrogen and oxygen. The overall efficiency of such PEC cells would be determined by the basic working principles and properties of photoactive materials. The tremendous progress made in the field of nanostructured materials may provide new opportunities for efficiently harnessing this technique.
Water Splitting with Nano-Sized Photocatalysts
As distinct from bulk photocatalysts, realized as thin films on conducting substrates, water splitting with nano-sized photocatalysts simply utilizes a photocatalyst material immersed in water. The principles of photocatalytic water splitting require high surface areas for electron excitation and collection, and the use of oriented nanocatalysts, which offer high surface to volume ratios and high light harvesting efficiencies, is a favorable match. Semiconductor nanocrystals can improve photocatalysis through the combined effects of quantum confinement and unique surface morphologies. A wide range of metallic nanoparticles can be prepared with adequate control over their size and shape. Nanoparticles can serve as building blocks for complex thin film structures. In particular, they self-assemble into ordered arrays—monolayers and multilayers—under specific conditions.
For preparation of such ordered nanoparticle arrays various deposition methods have been elaborated up to now. As a result of quantum confinement, materials that are not suitable semiconductors in bulk form due to insufficient energetic electrons or holes can be utilized on a nano scale. Surface and orientation modification of nano-sized catalysts may affect redox potentials and may be used to enhance the efficiency of charge transfer and charge separation. Furthermore, the problem of poor carrier transport in some bulk materials can be significantly alleviated on a nano scale, as the distance that photo generated carriers have to travel to reach the surface is significantly decreased.
There is still a need for improvement in this field, including the need for development of improved materials and devices that may operate with higher energy conversion efficiency.